In view of the recent instability of the price of crude oil and natural gas, there has been renewed interest in alternate sources of energy and hydrocarbons. Much of this interest has been centered on recovering hydrocarbons from solid hydrocarbon material such as oil shale, coal, and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily useable gaseous and liquid hydrocarbons.
Vast reserves of hydrocarbons in the form of oil shales exist throughout the United States. The Green River formation of Colorado, Utah, and Wyoming is a particularly rich deposit and includes an area in excess of 16,000 square miles. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States, almost sixty percent located in the Green River oil shale deposits. The remainder is largely contained in the leaner Devonian-Mississippi black shale deposits which underlie most of the eastern part of the United States.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be readily recovered by simple extraction. Upon heating oil shale to a sufficient temperature, however, kerogen can be thermally decomposed to liberate vapors, mist, or liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane, and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. After such a process, however, a carbon residue typically remains on the retorted shale.
Some well-known processes of surface retorting are: N-T-U (Dundas Howes retort), Kiviter (Russian), Petrosix (Brazilian), Lurgi-Ruhrgas (German), Tosco II, Galoter (Russian), Paraho, Koppers-Totzek, Fushum (Manchuria), Union B, Chevron STB, gas combustion and fluid bed. Process heat requirements for surface retorting processes may be supplied either directly or indirectly.
Directly heated surface retorting processes, such as the N-T-U, Kiviter, Fusham and gas combustion processes, rely on the combustion of some form of fuel, such as recycled gas or residual carbon in the spent shale, with air or oxygen within the bed of shale in the retort to provide sufficient heat for retorting. Directly heated surface retort processes usually result in lower product yields due to unavoidable combustion of some of the products and product stream dilution with the products of combustion. The Fusham process is shown and described at pages 101-102, in Oil Shales and Shale Oils, by H. S. Bell, published by D. Van Norstrand Company (1948). The other processes are shown and described in the Synthetic Fuels Data Handbook, by Cameron Engineers, Inc. (second edition, 1978).
Indirectly heated surface retorting processes, such as the Petrosix, Lurgi-Ruhrgas, Tosco II and Galoter processes, utilize a separate furnace for heating solid or gaseous heat-carrying material which is injected, while hot, into the shale in the retort to provide sufficient heat for retorting. In the Lurgi-Ruhrgas process and some other indirect heating processes, raw hydrocarbon-containing solid, e.g., oil shale or tar sand, and a hot heat carrier, such as spent shale or sand, are mechanically mixed and retorted in a screw conveyor. Such mechanical mixing often results in high temperature zones conducive to undesirable thermal cracking of vapor product as well as causing low temperature zones which result in incomplete retorting of the hydrocarbon-containing solid. Furthermore, in such processes, the solids gravitate to the lower portion of the vessel. Thus, stripping the retorted shale with gas causes lower product yields due to adsorption of a portion of the evolved hydrocarbons by the retorted solids. Generally, indirectly heated surface retorting processes result in higher yields and less dilution of the retorted product than directly heated surface retorting processes, but at the expense of additional materials handling.
Surface retorting processes with moving beds are typified by the Lurgi coal gasification process in which crushed coal is fed into the top of a moving bed gasification zone and upflowing steam endothermically reacts with the coal. In such a process, a portion of the char combusts with oxygen below the gasification reaction zone to supply the required heat of reaction. Moving bed processes are generally disadvantageous because the solids residence time is usually long, necessitating either reactors with very large contact or reaction zones or a large number of smaller reactors. Moreover, moving bed processes often cannot tolerate excessive amounts of fines.
Surface retorting processes with entrained beds are typified by the Koppers-Totzek coal process in which coal is dried, finely pulverized and injected into a treatment zone along with steam and oxygen. The coal is rapidly partially combusted, gasified, and entrained by the hot gases. Residence time of the coal in the reaction zone is only a few seconds. Entrained bed processes are disadvantageous because they require large quantities of hot gases to rapidly heat the solids and often require the raw feed material to be finely pulverized before processing.
Fluid bed surface retorting processes may, depending on the properties of the feed and the processing requirements, be particularly advantageous. The use of fluidized bed contacting zones has long been known in the art and has been widely used in the fluid catalytic cracking of hydrocarbons. When a fluid is passed at a sufficiently high velocity, upwardly through a contacting zone containing a bed of subdivided solids, the bed expands and the particles are buoyed and supported by the drag forces caused by the fluid passing through the interstices among the particles. The superficial vertical velocity of the fluid in the contacting zone at which the fluid begins to support the solids is known as the "minimum fluidization velocity." The velocity of the fluid at which the solid becomes entrained in the fluid is known as the "terminal velocity" or "entrainment velocity." Between the minimum fluidization velocity and the terminal velocity, the bed of solids is in a fluidized state and it exhibits the appearance and some of the characteristics of a boiling liquid. Because of the quasi-fluid or liquid-like state of the solids, there is typically a rapid overall circulation of all the solids throughout the entire bed with substantially complete mixing, as in a stirred-tank reaction system. Such rapid circulation is particularly advantageous in processes in which a uniform temperature and reaction mixture is desired throughout the contacting zone.
Many of these kerogen recovery processes remove the minerals from the oil shale prior to retorting. This removal step is commonly known as beneficiation. Beneficiation can take the form of chemical separation, physical separation, or both. Chemical separation can include leaching of mineral matter or extracting kerogen. Physical separation can include separating kerogen from mineral matter based on density, size, or surface property differences between kerogen and mineral matter. Although the specific gravity difference between pure kerogen and mineral matter can be as high as 2.8 g/cm.sup.3, taking advantage of this difference by isolating pure kerogen from mineral matter is very difficult and expensive because the kerogen exists in most oil shale as discrete deposits embedded in large amounts of mineral matter. Therefore, as a practical matter, the separation process involves separating kerogen-rich particles from mineral-rich particles. The density difference between kerogen-rich particles and mineral-rich particles is usually less than about 1 g/cm.sup.3.
Many methods of separating kerogen-rich particles from mineral-rich particles require that the oil shale be comminuted to a fine particle size in order to achieve an effective separation. Two examples of such methods are froth flotation and oil agglomeration. Froth flotation involves mixing the comminuted oil shale particles with an aerated aqueous solution. The aqueous solution contains a frother which reduces the surface tension of the solution, thereby producing a froth, and a collector to facilitate absorption of air bubbles at the kerogen-rich surfaces. The air bubbles preferentially absorb at kerogen-rich surfaces which have a greater hydrophobic character than mineral-rich surfaces. Absorbed air bubbles decrease the apparent density of the kerogen-rich particle, thereby causing them to float. A typical maximum particle size of oil shale particles treated by froth flotation is about 32-350 mesh screen size. Oil agglomeration involves mixing oil shale particles with a two phase liquid mixture of organic and aqueous phase. Kerogen-rich particles form agglomerates in the organic phase and mineral-rich particles form suspensions in the aqueous phase. A typical maximum particle size of oil shale particles used in oil agglomeration is about 100 mesh.
Because of the gum-like nature of kerogen, comminuting kerogen-rich particles to the fine particle sizes required by some of the beneficiation methods discussed above can be very expensive.
Spiral separators or concentrators are well known in the art for separating carbonaceous material from mineral matter in coal. This separation is based primarily on the specific gravity differentials of the carbonaceous material and the mineral matter. Raw coal is a physical mixture of carbonaceous material with a specific gravity range of 1.20 to 1.70 g/cm.sup.3 and mineral matter with a specific gravity range of about 5.0 g/cm.sup.3. Unlike oil shale which consists of discrete deposit of kerogen surrounded by a large amount of mineral matter, coal consists of a large amount of carbonaceous material surrounding a relatively small amount of mineral matter. In addition, the grain size of the mineral matter in coal is larger than the grain size of the kerogen in oil shale. As a result, separating the coal into its relatively pure carbonaceous and mineral components by comminuting the coal, forming a coal slurry, and separating these components from the slurry using a spiral separator has proven to be effective.